"Dancing with the Stars" celeb Jaleel White had a seriously bad rehearsal session with pro partner Kym Johnson on Friday -- leading to a "heated" confrontation in which Mark Ballas and show producers eventually intervened, an insider tells Us Weekly exclusively.

The "Family Matters" alum, 35, "flipped out," the insider says, after he accidentally stepped on Johnson's foot. When Johnson, 35, exclaimed "ouch" over his blunder, the insider says, White "got in her face," and chastised her for "acting like a baby ... He was shouting at her, calling her an 'idiot.'"

As Johnson momentarily left the studio in tears, fellow celeb competitor Donald Driver overheard the commotion and "tried to calm him down," the insider says. Unmoved, White "started going off on the producers and shouting about how they put him with a 'stupid' partner."

Producers then banished White from the studio for the rest of the day. "Now he says he refuses to go back to the rehearsal studio again, that he wants his own rehearsal space elsewhere," the source continues. "It was a huge fiasco that went down. A total mess."

A second show insider adds that Johnson "has never had a conflict with one of her dance partners before."